The document U.S. Pat. No. 5,915,285 describes a method for making a strain gauge for a transparent surface. This method of the prior art uses the piezoresistive property of indium tin oxide (In2O3—SnO2) or ITO, a transparent material that is deposited in the form of a thin layer on a substrate that makes up the surface to be functionalized. When said substrate is deformed, the thin layer of ITO is subjected to strain and its resistivity is modified. Thus, by measuring the variation in resistivity of said coating, the strain on the substrate can be detected. The piezoresistive effect of the ITO layer is measurable but corresponds to low resistivity variations. That resistivity variation depending on the strain defines gain, commonly known by the term of ‘gauge factor’. Thus, the gauge factor of a strain sensor that uses the piezoresistive effect of ITO is low, and precise detection of the mode of strain on the substrate makes it necessary to use measurement techniques and equipment whose cost is not compatible with that of devices intended for the general public.
The document WO 2012 016945 filed by the applicant describes the making of a tactile surface using conductive nanoparticles deposited in the form of an assembly of nanoparticles in colloidal suspension on said surface, particularly using a capillary/convective depositing method. The variation in conductivity of the assembly of nanoparticles under the effect of strain, which variation is attributed to conduction by tunnel effect between the nanoparticles of the assembly without being bound by any theory, makes it possible to obtain a gauge factor for the strain sensor made up in that manner that is much larger than what can be achieved using the piezoresistive effect. However, the making of such a strain sensor from nanoparticles of ITO or from other transparent nanoparticles available in the market does not provide satisfactory results. Firstly, the electrical conductivity of nanoparticles of ITO is too low to bring out the tunnel effect conduction mechanism sought by this type of gauge. Secondly, ITO nanoparticles are available in the market in the form of nanopowder, in which the size of the nanoparticles is very dispersed. The applicant has identified that the gauge factor obtained for a strain sensor as described in the document WO 2012 016945 is directly related to the homogeneity of the size of the nanoparticles contained in it. Besides, the introduction of nanopowder into a liquid phase in order to make the colloidal suspension leads to the formation of nanoparticle clusters, which further increase the dispersion of the size of the nanoparticles in the assembly. Finally, the capillary/convective depositing technique, which is industrially well adapted, also requires the suspension of nanoparticles that are homogeneous in size and sufficiently small. For example, the sensors described in the document WO 2012 016945 are made from mono-dispersed gold nanoparticles with a mean diameter of 15 nm (15×10−9 meter); this type of colloidal solution is commonly available in the market.